1. Field of the Invention
This invention relates to multiphase fluid flow measurement, and concerns in particular measurement of the rate of flow of a multiphase fluid mixture such as those commonly encountered in oil wells.
2. Description of the Prior Art
The fluids commonly encountered in oil wells are usually two- or three-phase fluids, being for the most part a mixture of water and oil or of water, gas and oil. One of the liquid phases--the water or oil--will be the continuous phase (usually water), and the other liquid phase, and the gas (if present), will be dispersed phases in the form of bubbles or droplets, often with a wide size spectrum, distributed within the continuous phase. In a hole which is inclined from the vertical, the multiphase fluid begins to separate--because the components are of different densities--into two (or more) distinct flow regions, with the lighter oil and gas lying above the heavier water. Moreover, these flow regions move with significantly different velocities, so that a substantial velocity gradient can be set up across the bore, possibly even such as to cause an actual backflow in those regions immediately adjacent the underside of the casing (and because this situation is unstable, large structures can be formed which also travel along the bore). The net result is that flow meters comprising a turbine/propeller blade driven by the moving fluid which are commonly used in oil wells, experience contradictory forces across the diameter, and the output of a flow measuring system relying on such a device becomes inaccurate.
It has been proposed to avoid this problem by adopting a different approach to measuring such flows. This method relies on there first being prepared, either by calculation or by direct measurement, information defining the velocity profile over the entire cross-section of the casing, one such profile for each of a suitable range of casing deviations and multiphase mixture proportions. Then, for a real casing of a known deviation through which is passing a multiphase fluid of known composition, there are taken a number of discrete velocity measurements--that is, measurements of the fluids passing a detector device which has a sensing area which is substantially smaller than the cross-sectional area of the casing having a known location in the casing's cross-section--and after selecting the profile corresponding to that deviation and composition the resulting actual velocity measured at the known point is used to deduce the actual velocity distribution over the casing, and by integration over the whole area the fluid flow rate is calculated. This method hinges on the ability to measure fluid velocity accurately at a location, and there are various types of detector system that have been proposed for this purpose. One particularly satisfactory system involves the use of thermal anemometry. A thermal anemometry device uses a wirelike filament through which is passed an electric current causing the filament to heat up. As the filament heats its electrical resistance changes, and by noting the current flow through the filament, the temperature of the filament can be measured. If a cold fluid is caused to flow past and in thermal contact with the filament then some of the filament's heat is conducted away and the filament becomes colder. Its resistance therefore changes, and this resistance change, after suitable calibration of the device with the relevant fluid, can be used to provide an accurate measure of the velocity of the fluid past the filament.
Although a thermal anemometer can be utilised to provide very acceptable figures for the flow rate of a single phase fluid, it is considerably less satisfactory when used with a multiphase fluid such as often found in an oil well borehole. The problem is caused by the dispersed phase, such as the oil, having one thermal characteristic, being dispersed within the continuous phase, such as the water, having a second thermal characteristic. While the continuous phase is interacting with the anemometer's filament the instrument gives one set of readings, but when a droplet of the dispersed phase material interacts with the filament then the readings change. It is not known for certain when the interaction is with one or the other materials, it is difficult to interpret the output of the instrument to provide an accurate measurement of the velocities of either phase.
It is an object of the present invention to provide a method of determining the velocity of a multiphase flow in which the problems outlined above are obviated or mitigated.